1. Field of the Invention
The present invention relates to the field of arrays of pixelated optical beam handling elements, especially to the use of such arrays in wavelength selective switches for use in optical communication systems, to methods of reducing the effect of pixel gaps on the wavelength performance of such switches.
2. Description of the Related Art
There exists a class of optical wavelength selective switches (WSS hereinafter) which depend on the dispersion of the wavelength components of an input signal along an array of beam modifying pixels, followed by an array of beam steering pixels, such as an array of MEMS. An example of such a switch is shown in International Patent Application Publication No. W02007/029260 for “Optical Wavelength Selective Router”, having a common inventor with the present application. In this example, polarization rotation by a pixelated Liquid Crystal (LC) array is used to modify the beams. Reference is made to FIG. 1, which illustrates schematically a top view of such a prior art WSS structure. The polarization diversified input beams 19 are wavelength dispersed in the plane of the drawing, conveniently by means of a diffraction grating 11. The wavelength dispersed beams are focused by a lens 12 onto a one-dimensional beam steering and switching array 13. In the exemplary switch shown in FIG. 1, a MEMS array 14 is used for the beam steering, and a pixilated liquid crystal cell 15 for attenuation of the switched beams. For the sake of simplicity, only three separate wavelength channels and three pixels are shown in FIG. 1, though it is to be understood that using an International Telecommunication Union (ITU) grid spacing of 100 GHz or 50 GHz (or any other spacing that may be used in the future), the number of channels that will fit into the bandwidth of the device will be much larger. The MEMS array steers the signals destined for different output ports in a direction out of the plane of the drawing, i.e. in the direction of the height of the switch, such that output signals are differently directed to enter different fiber optical collimators 10 shown in the side view of the collimator stack.
In such switches, there exists a potential problem because of the finite gaps between adjacent mirror pixels. That part of an optical beam falling on an inter-pixel gap is not directed back to its intended destination port, such that the transmission characteristic of the beam shows a notch of increased insertion loss at such gaps. So long as the channel grid associated with the pixels is the same as that of the pixelated array, this presents no problem, since the light associated with each channel allows enough band pass without any drastic loss changes and no light of interest will fall on the gap. However situations may arise where flexibility is required of the network, and it is desired to select the spectral width of some channels to be different from the effective wavelength width of a pixel, such as to designate some channels as having a higher information capacity and hence needing to carry laser modulation at a higher frequency. In such a case, the spread wavelength of the channel may fall on more than one mirror, requiring operation of two adjacent mirrors in unison to switch the channel, and that part of the width of the channels falling on an inter-pixel gap, and hence that part of the information at the wavelengths falling on the gap will be lost, or at least severely attenuated.
Reference is now made to FIGS. 2A and 2B which illustrate this problem schematically. FIG. 2A shows an array of pixels, typically a pixelated array of MEMS mirrors, each pixel having a width and grid spacing such that it is just filled by the width of the wavelength dispersed incident light falling on the array. In the example shown in FIG. 2A, an array for use with an ITU grid of 50 GHz is shown. The 50 GHz wide optical information channels are shown properly aligned such that each channel falls on a single pixel of the mirror array, with the center of each channel at 50 GHz, 100 GHz, 150 GHz, 200 GHz., etc., centered on the center of the pixels.
In FIG. 2B, there is shown a schematic plot of the transmission characteristic of the pixel array shown in FIG. 2A, plotted as insertion loss IL as a function of wavelength. As is observed, at each of the gaps between pixels, there is a sharp increase in insertion loss, corresponding to a frequency grid of 75 GHz, 125 GHz, 175 GHz, etc. Also shown in FIG. 2B, as shaded regions, is the useful bandwidth of each channel centered on the ITU grid. As is observed, since all of the light of each channel falls within a pixel, none of the information is lost or degraded because of impingement on one of the transmission dips at the gaps.
Reference is now made to FIGS. 3A and 3B, in which there is shown the same pixel array as in FIG. 2A, but for use in a situation for transmitting an incident channel of width 100 GHz, as would be desired for transferring a higher information content in the channel. The wider channel is centered at the 100 GHz ITU grid point. In FIG. 3B, in the insertion loss plot, there is shown a shaded region, denoting the useful bandwidth of the 100 GHz. wide channel, centered on the 100 GHz. grid point. As is observed, since the 100 GHz channel bandwidth now covers more than one mirror, (three in this case), transmission dips resulting from the gaps in the mirror array at 75 GHz and 125 GHz now fall within the bandwidth of the channel. As a result of this, information at those points is lost, or seriously attenuated.
Although the insertion loss of a single traverse of such a MEMS array mirror by itself may not be large enough to result in serious loss of information from the channel, it should be evident that in a complex switching network, where the signal may pass through a number of nodes all of which lie on the same grid such that all of the gaps fall at exactly the same wavelength, the gap loss at each node will be cumulative. As a result, the depth of the loss spikes at the gaps will be multiplied by the number of nodes traversed by the signal, such that after several such traverses, the loss at each gap wavelength could be catastrophic to the information content of the channel. Reference is now made to FIGS. 3C and 3D which illustrate this effect. FIG. 3C shows a schematic plot of the insertion loss of the first channel of information shown in FIG. 3B showing a sharp increase in insertion loss at the gaps located at 75 GHz and 125 GHz, while FIG. 3D shows the multiplied loss characteristic from traverses through multiple nodes of the system.
One method of reducing the effect of the gaps is by constructing a MEMS array having much higher mechanical accuracy, such that the gap can be made much smaller. Thus, whereas 6μ is a typical gap size of a MEMS array for use in such switches in the optical communication bands, if the gap size could be reduced to 0.5μ, there would be little interference with information transfer. However, such a narrow gap would impose serious mechanical tolerance problems on the manufacture of the MEMS array, and even if produced, such an array may be prone to mechanical malfunction. This solution is therefore generally impractical.
There therefore exists a need for a method of reducing the effect of inter-pixel gaps, such that at least some of the disadvantages of such prior art WSS's and systems can be overcome.
It is to be understood that the limitations generated in optical communication systems because of the presence of the finite gaps between the mirrors of a pixelated MEMS array are not limited to LC controlled WSS's, as described hereinabove. Such WSS's using LC polarization rotation control are only one common example of the use of pixelated MEMS mirror arrays, and it is to be understood that the problem arises with any application where MEMS mirror arrays are utilized, whether one-dimensional or two-dimensional.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.